Nope, neither reading nor writing to it. I've copied the file out of the Windows directory into another program's soundfont library, I'm reading and tryint to write to that location (C:\Users\Me\Documents\SomeProgram\Soundfonts)

You are using the latest version, I assume.
Error code 17 is a general "write file error". It arises when Viena tries to rename the temporary file written using the desired file name. The causes can be: folder is write protected, file is write protected (for example, open and locked in another application). Or, the folder or file name is bad - containing illegal characters. Normally, if the file is write protected, Viena tries to change the temp file to a new name including an index, e.g. gm(1).sf2 if gm.sg2 is protected. If this does not succeed then we have error code 17.

I tested the functionality just a second ago and it works for me, so...